1. Field of the Invention
The present invention relates to a spool valve for thick matter pumps according to the introductory part of claim 1.
2. Description of the Prior Art.
The invention relates in particular to thick matter pumps which deliver concrete. The inventive spool valve is suitable for two-cylinder thick matter pumps, whereby two spool valves may be disposed in its valve casing which run forward and backward in push-pull action. Each valve then controls a cylinder of the pump in such a way that said cylinder sucks in the thick matter in one end position of the spool valve and presses it into a feed pipe in the opposite position of the valve. Since the valves generally have the same design as each other, a description of one of these valves suffices.
The forward and backward movement of the valve requires the transfer of kinetic energy in both directions. For this purpose, working pistons are provided which are preferably driven hydraulically by a pressure medium. Both ends of the slide rod are directed out of the casing, which is advantageous, for instance, in that the faces of the slide rod cannot be blocked in the casing with caking thick matter. It is necessary to seal the slide rod in order to retain the liquid component of the thick matter, which is built up with fine particles, in the casing, in which the thick matter is subjected to considerable overpressure under the influence of the pumping process. Since the slide rod cannot be closed off, or at least not completely closed off, in the casing from the thick matter, it leads solid particles outside through the seal during its forward and backward movement. It is therefore necessary to clean the ends of the slide rod protruding out of the valve casing.
The point of departure for the invention is a known spool valve for thick matter pumps (German "auslegeschrift" No. 27 00 800). Herein, the distance between the driving cylinders and the valve casing is used for housing so-called "couplings" which connect the parts of the drive which move back and forth with the ends of the slide rod and are effective in both directions. This leads to outer dimensions which considerably exceed those of the working pistons or the working cylinders. The described operative connections between the working pistons and the slide rod can therefore not be housed in extensions of the working cylinders. However, this is possible in the case of other known spool valves for thick matter pumps which act on both ends of the slide rod with driving cylinders of simple action (German "auslegeschrift" No. 19 05 706). Herein, the cylinders are flanged directly onto the valve casing by the aid of a tubular extension, so that there is a free space between the cylinder face and the valve casing. The tubular extension of the working cylinders takes up the seal disposed outside on the casing and two bushings separate therefrom and adjacent one on the outside and one on the inside as a valve guide means. This creates in the tubular extension between the outer guide bushing and the working piston a cylinder chamber which grows larger and smaller in the rhythm of the working cycles and through which an oil liquid is directed which cleans the slide rod behind the seal.
In the spool valve forming the pint of departure of the invention, however, the seals are disposed in the guide bushes which have flushing chambers through which a flushing liquid also circulates. For it has turned out that fine grain from the thick matter is directed outside between the guide surface of the bushing and the slide rod. In the case of concrete pumps, this is usually so-called cement paste, a substance which also sets under water. The flushing liquid, which usually consists wholly or partly of oil, is supposed to clean the end of the slide rod, i.e. detach and take away the particles which have been carried along. However, the process is not perfect under practical conditions. This leads as a result to small amounts of solids, depending on the type of thick matter, getting into the outer guide means after they have overcome the seal, where they cannot be completely removed. Due to the numerous cycles of movement which the valve must perform during an operation of a thick matter pump, so much solid matter finally collects in the guide means that the slide rod may be blocked, but at least becomes sluggish. In spite of the great efforts for supplying and removing the flushing liquid, the cleaning effect is therefore insufficient.
In so far as the resulting difficulties multiply, the seals and guide means must be replaced. For dismounting and subsequent reassembly, the couplings, as the operative connection between the working piston and the protruding end of the slide rod, must be detached, which constitutes time-consuming work. Only then can the parts surrounding the slide rod be assembled and dismounted through the space between the cylinders and the pump casing. The subsequent reassembly of the couplings means a further considerable delay in completing the maintenance work or repair.
The invention is based on the problem of simplifying the structure of the spool valve and preventing it from being impaired by solids which are carried along by the slide rods.
This problem is solved by the invention with the features of claim 1. Expedient embodiments of the invention are the subject of the subclaims.